The power use of the robot is minimal compared to other appliances and lighting. Saving this cost could be a rounding error in the utility bill. Charging will consume maybe one ampere hour or two? When not charging the power load will be too small to show at all, and maybe nothing measurable on the wall plug -- charger off.Battery powered robots have minimal power compared to regular kilowatt powerful vacuums. They depend more on the brush than suction.

You could just leave the dock unplugged, in which case there is a slow battery drain from the computer alone -- around 0.1 amp, significant over time on the battery charge (not the utility bill). The battery switch or the Shut Down LCD menu selection cuts off. Probably good only starting the cleaning immediately when removing the dock, to keep a good charge.

The only other way might be putting a power timer, from hardware stores, on the wall plug for the dock, if some coordination with the programmed cleaning schedule can be worked out. Maybe set the dock on only in the low cost periods.

I am assuming the robot scheduling is functional when off the dock -- software might disable it, unknown, needs testing (anything in the manual?).